QUADRATE AND TYMPANIC RING In connexion with the elaboration of the chain of auditory ossicles it is very usual for mammals to possess a thin inflated bone, sometimes partly or entirely formed out of the tympanic bone, and known as the tympanic bulla. Whether this struc- ture is thin and inflated or thick and depressed in form it is characteristic of the mammals, and does not occur below them in the series. But it is not pre- sent in all mammals. It is absent, for example, in the Monotremes. When it is present it is some- times formed from other bones, as, for instance, from the alisphenoids. The tym- panic ring has been held to be the equivalent of the quadrate. It is more prob- FIQ 16._Under S1,rface of t?ie cranium of a Dog. a/pf, Anterior palatine foramen ; as, pos- ably the quadrato-jugal.1 Ribs. — All mammals are furnished with ribs, of which, the number of pairs differs considerably from group to group, or it may be even from species to species. The ribs are attached as a rule by two heads, of which one, the capitulum, arises as a rule between two centra of successive vertebrae. The terior opening of alisphenoid canal; AS, alisphenoid ; BO, basioccipital ; j8>S, basi- spienoid; cf, condylar foramen,; earn, ex- ternal auditory meatus ; JSss.O, exoccipital ; Jim, foramen "lacerurn. medium ; jft/pt foramen lacerum postering; fin>, foramen magnum; fa, foramen ovale ; fr, foramen rotundum ; JFV, frontal ; gf, glenoid fossa ; g$>, post- glenoid process ; Ma, malar ; Mscy maxilla ; oc, occipital condyle ; opy optic foramen ; Per, mastoid portion of periotic ; pgf, post- glenoid fossa ; JPl, palatine ; PJtfa:, pre- maxilla ; ftp, paroccipital process ; ppf, pos- terior palatine foramen ; PSt presphcnoid ; Pt, pterygoid ; #f, Kphenoidal fissure or fora- men lacerum aiiterius ; sm, stylomastoid fora- men ; /SO, supraoccipitai j Sg, zygornatic process of squamosal ; Tyt tympanic bulla; Fa, vomer. (From Flower's Osteology.) other, the tuberculum, springs from the transverse process. Only in the Monotremes 1 Gegenbaur, Vergl. Anat. Wvrbetth. Leipzig, 1898, p. 404.